1. Field of the Invention
This invention relates to a lead frame used for a semiconductor device and a process of producing such a lead frame.
2. Description of the Related Art
It is well known in the field of semiconductor devices that, because lead frames now include a large number of pins, it is necessary for finer inner leads and for shorter distances between these inner leads. In the prior art, a lead frame which includes a large number of such inner leads is usually formed by mechanical punching or chemical etching.
However, the inner leads of the lead frame formed by punching are not very fine and the distances between these inner leads cannot be as small as those formed by etching. Therefore, in many lead frames, the inner leads, and the like, are formed by etching so that the inner leads can be made fine and the distances between these inner leads can be made small.
However, if the inner leads are very fine and the distance between these inner leads is very small, the inner lead would more easily be deformed during the etching process and any subsequent process, such as a plating process.
The method for producing a lead frame known in the prior art, such as that described in Japanese Unexamined Patent Publication (Kokai) No. 2-210854, teaches that the tips of the inner leads are mutually connected by a connecting portion until certain processes (such as bending of the inner leads or the like) are carried out, and then the connecting portion is cut by punching. According to this method, it is possible to prevent the inner leads from being deformed until the processes are accomplished, even if such a lead frame is formed by etching.
However, a prior lead frame and a method of producing such frames, known in the prior art, have the following problems.
When a plurality of inner leads are formed by etching, so that the inner leads are arranged side by side to constitute a co-planar structure, each of the inner leads defines, in section, a trapezoid, which includes a lower side defined by a bonding surface and an upper side defined by a non-bonding surface, wherein the lower side is wider than the upper side. Since the distance between these inner leads is very small, burrs often remain after the inner leads are cut. These burrs may cause a connection between the adjacent inner leads and thus a short circuit therebetween.